One of conventional techniques for joining metals of different types or the same type together is cold welding which applies a load to members to be joined (hereinafter referred to as junction target members) to join the junction target members together (see, for example Patent Document 1, Japanese Patent Publication No. S59-052031).
Conventional cold welding will be described hereinafter with reference to FIGS. 8A-8C.
First, as illustrated in FIG. 8A, a plate member 2a whose front and back surfaces are coated with plating layers 8a and a plate member 2b whose front and back surfaces are coated with plating layers 8b are stacked. The plating layers 8a and 8b are made of nickel (Ni), and the plate members 2a and 2b are made of copper (Cu).
Next, as illustrated in FIGS. 8B and 8C, each contact surface of a dice 6a is brought into contact with the plate member 2a with the plating layer 8a interposed therebetween, and a pressure is applied to the plate member 2a in the direction indicated by the arrow A. Similarly, as illustrated in FIGS. 8B and 8C, each contact surface of a dice 6b is brought into contact with the plate member 2b with the plating layer 8b interposed therebetween, and a pressure is applied to the plate member 2b in the direction indicated by the arrow B. At this time, since copper has ductility higher than nickel, each of the plating layers 8a and 8b that are in pressure contact with each other cannot follow a plastic flow of the plate member 2a or 2b, and reaches a breaking point to be divided. Consequently, a pressure contact portion 5 formed by a bare surface is formed on regions of the plate members 2a and 2b where the plate members 2a and 2b are divided. Here, even if an oxide film is interposed between each of the plating layers 8a and 8b and an associated one of the plate members 2a and 2b, the oxide film follows the division of each of the plating layers 8a and 8b and moves, thereby exposing the bare surface on each of the plate members 2a and 2b. 
Thus, the plate members 2a and 2b can be joined together by providing plate members 2a and 2b with the plating layers 8a and 8b having lower ductility than the plate members 2a and 2b without removing oxide films formed on the plate members 2a and 2b to be in pressure contact with each other.
There is also a junction technique of replacing an oxide film removal solution with an oxidation inhibitor (see, for example, Patent Document 2, Japanese Patent Publication No. 2006-334652). In Patent Document 2, in joining metals at least one of which is made of copper, a junction surface of copper is brought into contact with the oxide film removal solution to remove an oxide film on the junction surface. Then, the junction surfaces of metals to be joined are brought into contact with each other with the oxide film removal solution remaining on the junction surface of copper, and are subjected to heat and pressure, thereby joining the metals together with the oxide film removal solution replaced with an oxidation inhibitor.